The advantage of using carbon fiber reinforced epoxy or other thermosetting or thermoplastic resins in advanced structures, especially for aircraft, are well known in the industry. In recent years, carbon composite materials have begun to find wide acceptance in aircraft structures. With this new material have come new manufacturing, maintenance and life cycle management processes as well as machines and analysis methods to manufacture these materials.
A composite material consists of fibrous material, usually glass or carbon, embedded in a resin matrix. This matrix is usually a thermosetting or thermoplastic resin. Resins are often pre-impregnated into the fiber in an optimum fiber to resin ratio forming what is termed a pre-preg. In this form, the pre-preg can be stored in a refrigerated environment and will be viscous and tacky with a consistency of cold molasses. Composite structures are made of layers of this pre-preg fabric. During the lamination of several layers, air can become entrapped between adjacent layers leading to small voids in the final cured material that can lead the failure of the material under load.
The most common method of reducing voids in a composite laminate is to use what is termed a vacuum bag. In this process a bag is placed over the finished lamination and the bag is then vented to a vacuum, effectively applying one atmosphere (about 14.7 psi) of pressure to the surface of the laminate, compressing it against the tooling surface. A schematic of this process can be seen in Prior Art FIG. 1. Most high quality resin systems require elevated temperatures to complete the polymerization or curing of the resin. As such, the vacuum bagging process is commonly used in conjunction with an oven that heats the vacuum bagged laminate to cure it.
Typical curing of an aircraft quality composite laminate uses a combination of a vacuum bag to help remove trapped air from the laminate, and an autoclave (a pressurized oven) to decrease the size of trapped air voids in the laminate. This dual system is used to assure higher quality laminates than the vacuum and oven combination alone. A vacuum bag is applied to the laminate after final lamination is complete. In the case of thick laminates, the vacuum can be applied at intervals along the way to decrease the overall volume of trapped air in the laminate. The laminate is then placed in an oven and heated at a time and temperature cycle that usually involves a ramp and hold sequence specific to the resin chemistry. As it is heated, the laminate will soften and the resin will become less viscous. During this window of time, before significant resin polymer cross-linking occurs, the vacuum and the autoclave work in conjunction to remove and compact voids in the lamination. Some amount of void development also occurs as a result of volatile evaporation in the resin during curing. A schematic of this vacuum bag in autoclave process can be seen in Prior Art FIG. 2.
The high pressures involved in the autoclave process (approximately 100 psi) reduce the void content in the laminates and therefore provide long fatigue life airframes for decades of reliable service and tens of thousands of pressurization cycles and flight hours carrying millions of passengers. The manufacture of composite airframes (such as the Boeing 787) in production quantities is substantially dependent on the use of autoclaves which have been in extensive use for the last 4 decades.
The autoclave is a pressure vessel and an oven. As a pressure vessel, the primary cost discriminator for an autoclave is its diameter. An incremental increase in diameter will cost more than an equivalent increase in the length of the autoclave. For large manufacturing operations an autoclave represents a large portion of the manufacturing machines and tooling costs associated with composite manufacturing. Although autoclaves have been produced at diameters up to 30 feet, and lengths nearing 100 feet; for many manufacturers the multi-million dollar investments required to produce autoclave parts this size are prohibitive. Prior Art FIG. 3 illustrates a large autoclave. Wider use of composites has meant an increase in part complexity and size, further pushing up the required sizes of autoclaves. The setup and process time associated with using an autoclave also represents a significant portion of the manufacturing process time of a composite part. Although several resin systems used in conjunction with autoclaves have been approved for use in aircraft primary structure, the cost and schedule impact of a large autoclave is a significant impact on a manufacturing operation.
Void elimination from the vacuum bag/autoclave process is not 100% achievable. One major contributor to this is the evolution of volatiles (and in some cases reaction by-products like water) from the resin during curing. As this evolution is occurring, the resin is cross linking and becoming a solid, reducing the ability to migrate the by-products through the laminate. The composite manufacturer is limited in their process window by the resin chemistry. Simply curing at higher temperatures, or longer cycles with longer intermediate hold profiles does not produce the same end state cured matrix. Over curing of the resin can result in a brittle or weaker resin.
In U.S. Pat. No. 7,186,367 B2 to NASA, a solution to the time-temperature vs. void content limitations in resin curing is proposed. The double vacuum process described in the prior art involves curing the resin under two vacuums. Instead of achieving a surface pressure of one atmosphere in the single vacuum process, the inventor suggests the use of a vacuum both above and below the laminate. This effectively allows the laminate to rest loosely in a vacuum environment. This non-compacted state allows for less restricted egress of volatiles and air voids in the laminate. The resin is heated slowly through ramp up cycle that allows the viscosity to drop while the double vacuum condition exists. As the laminate reaches the hold temperature, where the bulk of resin curing will occur, the vacuum existing above the vacuum bag is allowed to collapse, causing compaction of the laminate through the typical single vacuum process.
A schematic of this process is shown in Prior Art FIG. 4. A composite laminate 12 is on the mold tool 16 with the appropriate perforated peel ply and bleeder cloth 14. Sealing tape 20 is placed on the tool around the laminate with a port venting to vacuum 22 within the boundary of the tape. The first vacuum bag 18 is then placed over the laminate and sealed on the double sealing tape 20. For larger parts the assembly is then placed in a vacuum oven for application of the outer vacuum. For smaller parts, a perforated and rigid housing 34 is then placed over the part and first vacuum bag. A second vacuum bag 26 is placed over the rigid housing and sealed with double sealing tape 30. Within the bounds of this sealing tape and outside the bounds of the first bag is another vacuum port in the mold tool 28. An initial vacuum is then pulled through the first port 22 and second port 28. As the curing cycle ramp levels off to the high temperature hold where cross linking occurs; the second vacuum port 28 is vented to atmosphere, applying ambient pressure the first bag 18.
The recent development of out-of-autoclave resin systems (such as MTM-45™, or Cycom™ 5215) is of great interest to the composite industry. These new resin systems promise to deliver the material properties and void contents of traditional autoclave cured laminates without the time and expense of the autoclave process. Both MTM-44™ (already approved for Airbus™ commercial secondary structures) and MTM-45™ are out-of-autoclave resin systems that compare well with FAA approved autoclave cured resin systems already in use in aircraft primary structure shown by the diamond labeled points. Combining a void decreasing non-autoclave process such as the double vacuum method with an out-of-autoclave resin system will provide the necessary quality increase to create an out-of-autoclave process with sufficient quality to be approved for primary structures.
These out-of-autoclave resin systems are intended to be used in conjunction with a single vacuum bag process and an oven. This represents a significant labor and cost savings with the elimination of the autoclave. However the traditional autoclave process has a long history of processing standardization and material properties databases that are trusted in the aircraft industry. In a necessarily conservative industry, the acceptance of out-of-autoclave resins has been slow and these resin systems have not yet been accepted by military customers and the civilian certification authorities (FAA, JAA, CAA, etc.) for the manufacture of primary aircraft structures. Of primary concern to these oversight authorities are the reliability of the material properties resulting from out-of-autoclave resins. The process sensitivity to time temperature profile variations and vacuum pressures is one such concern.
Prior Art FIG. 5 illustrates the relative glass transition temperature of various resin systems versus their impact strength after compression. Glass transition temperature is a measure of viable operating temperature of a resin before significant reduction in material properties. Compression strength after impact is a measure of damage tolerance of the matrix material.
Thus, there is still a need for improvements in the field, especially with respect to highly stressed thick laminates and/or in manufacturing of complex laminates with highly stressed joints in a co-cured structure.